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Old 01-27-2018, 12:50 AM   #1
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Default D21 VG30 Ignition coil bad

Accidentally left the ignition switch on for about 2 hours. That not only drained the battery, but burned up the coil. The coil was hot as a firecracker.

I had a spare coil so I was able to diagnose it immediately.

As everyone knows, these coils are very small. I never thought there would be enough power going though the coil that way to burn it up.

Makes me wonder if I could substitute a full sized electronic ignition (low impedance) coil?

I also have to wonder, because of the heat, if the coil isn't susceptible to degrading over time even when in normal use?

This is the second time over the life of the truck I have replaced the coil. Last coil was OEM, this time I am using aftermarket which has exposed core laminations. Maybe it will dissipate heat better because of that.
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Old 01-27-2018, 01:31 AM   #2
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I would stay stock. The stock coils are stupid hot. Going aftermarket will actually result in a colder spark. You could always do something like a turbo timer to kill the ignition if you forget to.
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Old 01-27-2018, 02:15 AM   #3
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Thanks for the suggestion.

I tested the damaged coil cold and the primary and secondary resistances are within specifications: 1.0 ohms on the primary and 9,600 ohms on the secondary.

This proves the coil can spec out fine but yet be defective.

I don't see any reason for the laminated core to be encapsulated by plastic as the OEM coils are. That would just make the coil hotter, thus shorter-lived and maybe not quite as hot a spark because heat makes resistance.

I am going to guess when I left the ignition "ON", the distributor was in the position to "Fire", thus the transistor switch on the module was grounding the primary.

Too bad there is not a sensor to turn the transistor switch off when the engine isn't running.

I would be curious to know how much current is going through the primary when it is getting power. If the primary input is 12V, and if the only resistance is the primary coil at 1 ohm, that would be 12 amps going through the primary, quite a drain on the battery. Of course, there might be a resistor in the ignition module to limit it to less. Still, whatever it is, there is enough to burn-up the coil.

I would like to know why the coil is so small. I compare it to my other vehicle, a VW, also from that time period, which has a much larger and heavier coil. Both electronic-type coils and ignitions.

Last edited by chickenfriend; 01-27-2018 at 02:21 AM.
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Old 01-27-2018, 04:53 AM   #4
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Rarely do I see something bad ohm out correctly. Where are you getting your numbers from? The FSM?

For some.reason I can't see the truck keeping the coil hot uses you left the key on ON and no accessory. Accessory should only do accessories.

Why make a large inefficient coil when you can make a small efficient one? The more I tear into my truck the more I see how much Nissan loved tight tolerances.
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Old 01-27-2018, 07:28 AM   #5
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Yeah, the key would have to be left in the ON position. I don't know much about all this, but if the coil just isn't working, toss it away and try another one.

Then remember to switch it off from now on. It's no big deal. It's like totally killing a battery and it won't recharge. Replace it, and then don't do it again.
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Old 01-28-2018, 01:27 AM   #6
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the VG30i coil isnt energized unless the distributor is turning and sending the signal generated by the wave plate.
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Old 02-14-2018, 12:50 PM   #7
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I saw a post on here about using a coil from a v8 dodge Dakota as an upgrade.
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Old 02-14-2018, 01:45 PM   #8
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Quote:
Originally Posted by Hardbody92 View Post
I saw a post on here about using a coil from a v8 dodge Dakota as an upgrade.
Thanks for the info. I tried searching for that post, but have no had any luck finding it so far. The coils have a similar appearance, looks like a different type of wire connection would be necessary for the ignition wire between the coil and the distributor cap.

That reminds me, I need to order a spare. I might try a low impedance VW coil to see if it works. First I would check to see the primary resistance values overlap in the same range--don't want to burn-out the electronic module.
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Old 02-14-2018, 02:13 PM   #9
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Quote:
Originally Posted by cadillacdude1975 View Post
the VG30i coil isnt energized unless the distributor is turning and sending the signal generated by the wave plate.
Yeah, I feel like I'd have burned mine out by now if the coil was running when the truck was in accessory. I've had my truck running for well over an hour in accessory-only when using it as a power source for my electric impact.
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Old 02-14-2018, 04:36 PM   #10
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Quote:
Originally Posted by OldGreyBeast View Post
Yeah, I feel like I'd have burned mine out by now if the coil was running when the truck was in accessory. I've had my truck running for well over an hour in accessory-only when using it as a power source for my electric impact.
I would be surprised if the coil were getting power in "Accessory Mode". I am sure, however, it is energized when the ignition key has been mistakenly left "ON".

My 84 Rabbit is the same way, coil is on when ignition is on. Engine does not have to be running; I don't think it has an Accessory Mode.

My point is that the design of the coil should be robust enough to handle being ON when the ignition is ON, without overheating without melting down the insulation on the coil wires, because it is occasionally possible to forget about the key being left "ON", especially if we are doing any type of engine work or doing checks.

To get the higher voltages in the secondary, with a smallish design, they would have had to have used smaller diameter wire in the secondary to get the proper turns-ratio. My guess is the primary wire is small diameter, too.

Also, it is possible for a coil to go bad and still ohm-out correctly, as I indicated with my experience just described, and what is common knowledge. With an ohm test, you are only putting,, what, 1/2 volt against the coil? That is not enough to predict what happens with much as four magnitudes more of voltage when it is operation.

Last edited by chickenfriend; 02-14-2018 at 04:42 PM.
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